kottke.org posts about Einstein

Put a spinning gyroscope into orbit around the Earth, with the spin axis pointed toward some distant star as a fixed reference point. Free from external forces, the gyroscope's axis should continue pointing at the star--forever. But if space is twisted, the direction of the gyroscope's axis should drift over time. By noting this change in direction relative to the star, the twists of space-time could be measured.

Gravity Probe B's experiment was 47 years in the making, helped spawn 100 PhD theses, and required the invention of 13 brand-new technologies, including a "drag-free satellite." The four gyroscopes in GP-B are "the most perfect spheres ever made by humans... If the gyroscopes weren't so spherical, their spin axes would wobble even without the effects of relativity."

NASA finished collecting the data in 2005; now they've crunched the numbers. And yes, Einstein was right. The gyroscopes wobble in just the way general relativity predicts.

The first and most famous empirical experiment testing Einstein's theory was performed in 1919 by Arthur Eddington during a full solar eclipse. Photographs showed that the sun's mass caused starlight to bend around it.